This application in general relates to a method for collecting data from a chemical sensor. The inventive method eliminates variations in monitored readings from the chemical sensors which may result based on variations in the composition of the chemical sensors, or based on station-to-station variations between several of the sensors and their associated monitoring devices.
Known chemical sensors are utilized in combination with a blood culture vial to monitor changes in the blood culture vial. As is known, a small quantity of blood is injected into a vial which contains a culture medium. A chemical sensor is positioned inside the vial. The vial is then incubated and monitored for bacterial growth.
Known chemical sensors include a variety of types of sensors which change their light absorption or fluorescent intensity based on changes within the culture medium. Radiation is directed into the chemical sensor, and the intensity of a resultant emission is monitored. The blood culture medium is tested by monitoring the emission over time to detect changes in emission intensity from the chemical sensor. If the blood sample is a "positive" and contains bacteria, then it is expected that the emission would change with time. In known systems, the emissions are monitored over a period of days to determine whether there is a change. The monitored emissions may be relatively static for a period of days, but may then change drastically to evidence the presence of bacteria in the sample. Such known systems have been widely utilized and have experienced great success.
Some deficiencies remain with these systems, however, since the testing systems for use with the chemical sensors are typically testing hundreds of sample vials at any given time. The monitored emission from any particular chemical sensor can vary with variations between the light source positioned adjacent to the sensor, the chemical sensor itself, and with a light detector positioned adjacent to the chemical sensor. Thus, an absolute determination of whether a particular vial is positive or negative upon any one given reading has not been possible with the prior art systems. Rather, the emission at any one point in time provides little information with regard to the status of the sample vial. The emissions must be monitored over a period of time to detect the changes in the emissions which indicate a positive sample.
Problems arise with sample vials which have been prepared from a relatively long period of time relative to the typical sample vial. As an example, a sample vial may be prepared on a Friday and tested on a Monday. In such so-called "late" sample vials, it is possible that the sample vial has already turned positive, that all changes have occurred, and that monitoring to detect changes in the emissions will be of no avail. There may be no further changes, and thus the monitoring will detect no changes which would give rise to an indication that the sample vial is positive.
Further, with the variation that is inherent in the prior art sensor readings, it is possible that a positive readings for a sample from a first sensor at a first station might be of values that are roughly equivalent to negative readings for other samples at other stations. For that reason, no reliance can be made on the absolute value of any reading. Given that fact, the above-described problem of a late vial could result in the failure to ever detect the late vial as being a positive.
It has been proposed to address the problem of station-to-station or sensor-to-sensor variations by taking two readings from each sensor based upon two distinct wavelength radiations being directed into the sensor. Such prior an systems have hoped to eliminate station-to-station or sensor-to-sensor variation by taking a ratio of those two readings. However, it has been found that variables within the station and within the sensor vary with varying wavelengths. Thus, the prior an use of a ratio based on two wavelengths has not fully addressed the above-discussed problems. The variation with the changing wavelengths is smallest when the change in wavelengths is kept to a minimum. However, when a small wavelength change is utilized, the change in emissions from the radiation directed into the sensor at the two wavelengths is also small between the two wavelengths. A straight ratio of two close wavelength emissions would typically be effectively equal to one, and no relevant information would be provided by the ratio. Thus, one typically cannot achieve any useful result using a straight ratio of the intensities of the emissions directed outwardly of the chemical sensor from two wavelengths. The prior an use of ratios does not eliminate variation in the sensors or equipment having an effect on the detected emissions.